U.S. patent number 5,636,614 [Application Number 08/605,796] was granted by the patent office on 1997-06-10 for electronic control system for an engine and the method thereof.
This patent grant is currently assigned to Fuji Jukogyo Kabushiki Kaisha. Invention is credited to Koji Morikawa.
United States Patent |
5,636,614 |
Morikawa |
June 10, 1997 |
Electronic control system for an engine and the method thereof
Abstract
The air-fuel ratio of a lean-burn engine is controlled so as to
be adjusted to a lean-burn air-fuel ratio with a low NOx
concentration. As this is done, an actual combustion fluctuating
rate is detected from the cylinder pressure, and the air-fuel ratio
is adjusted to the rich side with respect to the lean-side limit,
whereby a satisfactory driving performance can be secured. The
state of NOx exhaust is determined by an actual NOx concentration,
and the air-fuel ratio is adjusted to the lean side with respect to
the allowable limit of the NOx exhaust. On the other hand, the
combustion fluctuating rate and an allowable limit value of the
combustion changing rate under the driving conditions concerned are
compared, while the NOx exhaust rate and an allowable limit value
of the NOx exhaust rate under the same driving conditions are
compared. The EGR amount is decreased when the combustion
fluctuating rate is higher than the allowable limit value, and is
increased when the NOx exhaust rate is higher than the allowable
limit value of the NOx exhaust rate. Thus, the NOx exhaust can be
decreased.
Inventors: |
Morikawa; Koji (Tokyo,
JP) |
Assignee: |
Fuji Jukogyo Kabushiki Kaisha
(Tokyo, JP)
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Family
ID: |
26340762 |
Appl.
No.: |
08/605,796 |
Filed: |
February 22, 1996 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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355190 |
Dec 8, 1994 |
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Foreign Application Priority Data
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Dec 17, 1993 [JP] |
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317685 |
Jan 25, 1994 [JP] |
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6577 |
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Current U.S.
Class: |
123/435 |
Current CPC
Class: |
F02D
35/023 (20130101); F02D 41/1475 (20130101); F02D
41/005 (20130101); F02D 41/1461 (20130101); F02D
41/1454 (20130101); Y02T 10/40 (20130101); Y02T
10/47 (20130101); F02D 41/1498 (20130101) |
Current International
Class: |
F02D
21/00 (20060101); F02D 41/00 (20060101); F02D
41/14 (20060101); F02D 21/08 (20060101); F02D
041/14 () |
Field of
Search: |
;123/435 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Dolinar; Andrew M.
Attorney, Agent or Firm: Beveridge, DeGrandi, Weilacher
& Young, L.L.P.
Parent Case Text
This is a divisional of co-pending application Ser. No. 08/355,190
filed Dec. 8, 1994 pending.
Claims
What is claimed is:
1. An electronic control system for an engine having, an intake
manifold connected to said engine for inducing air and fuel
mixture, an airflow meter mounted on said intake manifold via a
throttle valve for measuring an amount of air induced thereof and
for generating an air amount signal, an exhaust manifold connected
to said engine for exhausting burnt gases, a nitrogen oxide
concentration sensor inserted in said exhaust manifold for
detecting a nitrogen oxide amount in said burnt gases and for
producing a nitrogen oxide signal, a crank angle sensor mounted on
said engine for sensing an engine speed and for generating an
engine speed signal, a pressure sensor mounted on said engine for
detecting a combustion pressure in a cylinder and for outputting a
pressure signal, an EGR valve communicated to said exhaust manifold
for recirculating exhaust gases to said intake manifold and mode
setting means for switching an engine operating mode from an
economy mode to a power mode or vise versa, the system
comprising:
driving condition determining means, responsive to said engine
speed and pressure signals, for deciding an operating condition of
said engine and for generating an operating condition signal;
combustion fluctuating rate calculating means, responsive to said
pressure signal, for calculating an actual combustion fluctuating
rate and for generating a fluctuating rate signal;
combustion fluctuating rate comparing means, responsive to said
operating condition and said fluctuating rate signals, for deriving
an optimum air-fuel ratio by comparing said fluctuating rate signal
with a standard value stored in a map and for producing a first
control signal;
nitrogen oxide gas calculating means, responsive to said nitrogen
oxide signal, for calculating an actual nitrogen oxide gas amount
and for generating a nitrogen oxide signal;
exhaust gas comparing means, responsive to said operating condition
signal and said nitrogen oxide exhaust signals, for judging said
optimum air-fuel ratio by comparing said actual nitrogen oxide gas
amount with a desired nitrogen oxide value stored in a memory and
for producing a second control signal; and
emission gas recirculation rate setting means, responsive to said
first and second control signals, for deciding an optimum EGR rate
by referring a target EGR rate corresponded to each engine
operating condition stored in an EGR map so as to accurately
operate said EGR valve in both said economy and power modes.
2. The electronic control system according to claim 1, wherein
said second control signal increases said EGR rate when said
nitrogen oxide amount is larger than said desired nitrogen oxide
value.
3. The electronic control system according to claim 1, wherein
said first control signal decreases said EGR rate when said
combustion fluctuating rate is higher than said standard value.
4. The electronic control system according to claim 1, wherein
said target EGR rate is decided based on said engine operating mode
when said actual combustion fluctuating rate is lower than said
standard value and when said actual nitrogen oxide gas amount is
smaller than said desired nitrogen oxide value.
5. A control method for an engine having an intake manifold
connected to said engine for inducing air and fuel mixture, an
airflow meter mounted on said intake manifold via a throttle valve
for measuring an amount of air induced thereof, an exhaust manifold
connected to said engine for exhausting burnt gases, a nitrogen
oxide concentration sensor inserted in said exhaust manifold for
detecting a nitrogen oxide amount in said burnt gases, a crank
angle sensor mounted on said engine for sensing an engine speed, a
pressure sensor mounted on said engine for detecting a combustion
pressure in a cylinder, an EGR valve communicated to said exhaust
manifold for recirculating exhaust gases to said intake manifold at
an EGR rate and mode setting means for switching an engine
operating mode from an economy mode to a power mode or vise versa,
the method comprising:
calculating a combustion fluctuating rate from a ratio between said
combustion pressure and a predetermined averaged value
corresponding to an engine speed and an amount of air;
computing a nitrogen oxide exhaust rate by comparing said nitrogen
oxide amount with said amount of air;
comparing said combustion fluctuating rate with a predetermined
fluctuating rate corresponding to an engine speed and an amount of
air;
judging whether said nitrogen oxide exhaust rate is larger than a
limit value when said combustion fluctuating rate is smaller than
said predetermined fluctuating rate;
increasing said EGR rate when said nitrogen oxide exhaust rate is
larger than said limit value and said engine operating mode is in
said economy mode; and
decreasing said EGR rate when said nitrogen oxide exhaust rate is
smaller than said limit value and said engine operating mode is in
said power mode .
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an engine control apparatus for
controlling a lean-burn air-fuel ratio of an air-fuel mixture in a
lean-burn engine for a vehicle, and more particularly, to the
engine control apparatus provided with an EGR (exhaust gas
recirculation) system in which a small amount of exhaust gas is
mixed with intake air, whereby reduction of nitrogen oxides in the
exhaust gas and improvement of the running performance can be
expected.
2. Description of the Related Art
Lean-burn engines have been studied and developed as fuel-saving
engines for new-generation vehicles. In this type of the engine,
swirl or turbulence is generated in a combustion chamber during air
induction, and a leaner air-fuel mixture than that of a theoretical
air-fuel ratio is burned. In such a lean-burn engine, the air-fuel
mixture is so lean that the amount of exhausted HC and CO gases is
originally small, whereas perfect combustion advances to increase
the NOx exhaust gases. After a certain air-fuel ratio is reached,
however, the NOx delivery decreases to improve the exhaust gas
characteristics as the air-fuel ratio increases.
Prior art air-fuel ratio control techniques are described in Jpn.
Pat. Appln. Laid-Open (KOKAI) Publication Nos. 60-27748 and
58-38354. In the case where the air-fuel ratio exceeds the
lean-side limit, there is a possibility of a misfire, an increase
of combustion fluctuation, and driving performance deterioration.
In the conventional methods, torque variation is detected so that
the air-fuel ratio is subjected to lean-limit control based on the
detected value of the torque variation, whereby the misfire and
lowering of the driving performance are prevented.
According to the techniques of the former publications, however, it
cannot be determined whether or not the NOx is actually decreased
while the engine is in an operating state, so that the air-fuel
ratio must be inevitably decided to a set value within leaner side
based on the characteristics of exhausted NOx concentration.
According to a technique described in Jpn. Pat. Appln. Laid-Open
(KOKAI) Publication No. 58-13137, moreover, the NOx concentration
is estimated indirectly from the cylinder pressure, and an EGR
system and the like are controlled in accordance with the NOx
concentration, whereby the NOx is decreased.
However, the air-fuel ratio control disclosed in the latter
publication is adapted for the EGR system and the like, and cannot
be applied to lean-burn air-fuel ratio control.
Conventionally, exhaust gas recirculation (EGR) control is widely
used as an effective method for restraining the formation of
nitrogen oxides at the time of combustion. In the EGR control, the
amount of the exhaust gas is mixed with the intake air, thereby
increasing heat capacity of the gases in the cylinder to lower the
temperature of the burning gas relatively. If the EGR amount is too
large, however, the combustion fluctuation is caused to lower the
output, fuel cost performance, and reliability of running
performance. It is generally known, therefore, that the EGR amount
must be restricted to a necessarily minimum value.
According to a technique described in Jpn. Pat. Appln. KOKAI
Publication No. 2-252958, for example, an EGR control region is
divided into two parts, feedback and open-loop control regions,
depending on the engine driving conditions. In the feedback control
region, a target EGR rate for each driving condition is set by
referring a map, using the accelerator pedal opening degree and
engine speed, whereby the EGR rate is controlled.
In the EGR disclosed in the publication, however, the target EGR
rate is set depending on the driving conditions. Although the EGR
control can be effected corresponding to the quantity of NOx
produced, therefore, it is impossible to restrain combustion
fluctuation which accompanies the decrease of the exhaust of the
NOx. Accordingly, satisfactory running performance cannot always be
obtained in the EGR control region.
Described in Jpn. Pat. Appln. Laid-Open (KOKAI) Publication No.
2-298657, moreover, is a technique such that the EGR amount is
controlled in accordance with the intensity of combustion light
emitted in the combustion chamber, whereby a sudden increase of the
combustion speed is avoided, and the generation of the NOx is
restrained.
According to this method, the EGR control is carried out depending
on the state of combustion which is detected by the light
intensity. Since the NOx exhaust is not detected, however, the EGR
rate is not accurately controlled.
SUMMARY OF THE INVENTION
A first object of the present invention is to provide an engine
control apparatus which can detect the combustion fluctuating rate
and NOx exhaust of a lean-burn engine, to keep the air-fuel mixture
within a proper lean-burn region, thereby improving the exhaust gas
characteristics and the driving performance.
A second object of the invention is to provide an engine control
apparatus which can restrain the combustion fluctuation and
reduction of the NOx exhaust, in the engine with the EGR system, so
that satisfactory control accuracy can be attained and the driving
performance is available at a driver's intention.
In order to achieve the first object, according to a first aspect
of the present invention, there is provided an electronic control
system for an engine having, an intake manifold connected to the
engine for inducing air and fuel mixture, an airflow meter mounted
on the intake manifold via a throttle valve for measuring an amount
of air induced thereof and for generating an air amount signal, an
exhaust manifold connected to the engine for exhausting burnt
gases, a nitrogen oxide concentration sensor inserted in the
exhaust manifold for detecting a nitrogen oxide amount in the burnt
gases and for producing a nitrogen oxide signal, a crank angle
sensor mounted on the engine for sensing an engine speed and for
generating an engine speed signal, and a pressure sensor mounted on
the engine for detecting a combustion pressure in a cylinder and
for outputting a pressure signal, the system comprising: driving
condition determining means, responsive to the engine speed and
pressure signals, for deciding an operating condition of the engine
and for generating an operating condition signal; combustion
fluctuating rate calculating means, responsive to the pressure and
operating condition signals, for calculating an actual combustion
fluctuating rate and for generating a fluctuating rate signal;
combustion fluctuating rate comparing means, responsive to the
operating condition and the fluctuating rate signals, for deriving
an optimum air-fuel ratio by comparing the fluctuating rate signal
with a standard value stored in a map and for producing a first
control signal; nitrogen oxide exhaust calculating means,
responsive to the nitrogen oxide signal and the operating condition
signal, for calculating an actual nitrogen oxide gas amount and for
generating a nitrogen oxide signal; exhaust gas comparing means,
responsive to the operating condition signal and the nitrogen oxide
signal, for judging the optimum air-fuel ratio by comparing the
actual nitrogen oxide gas amount with a desired value stored in a
memory and for producing a second control signal; and fuel
injection calculating means, responsive to the first and second
control signals, for deciding an optimum fuel injection amount
corresponded to each driving condition so as to accurately control
the engine without fluctuation.
With the arrangement described above, according to the present
invention, the air-fuel ratio of the lean-burn engine is controlled
to be adjusted to a lean-burn air-fuel ratio with less NOx.
Therefore, the actual combustion fluctuating rate is detected on
the basis of the internal pressure in the cylinder, and the
air-fuel ratio is controlled on the rich side with respect to the
lean-side limit, so that satisfactory driving performance can be
maintained. Moreover, the state of NOx exhaust is determined by the
actual NOx concentration, and the air-fuel ratio is controlled on
the lean side with respect to the allowable limit of the NOx
exhaust, so that the NOx exhaust can be securely decreased.
Thus, according to the present invention, the actual concentration
of the NOx in the exhaust gas in the lean-burn engine is detected
so that the state of NOx delivery can be determined, and the
air-fuel ratio is controlled so as to be within a region between
the lean-side limit for combustion change and the allowable limit
of the NOx exhaust. Thus, the driving performance can be improved,
and at the same time, the NOx in the exhaust gas can be decreased
securely. Moreover, the region for the air-fuel ratio control is
extended on the rich side, so that vibration can be also avoided in
the driving conditions.
Moreover, the combustion fluctuation is compared with the
predetermined value for each set of driving conditions, whereby the
proper operating condition is determined. Also, the exhausted NOx
is compared with the predetermined value to determine the state of
the exhaust gas, and the air-fuel mixture is controlled so as to be
richer or leaner. Thus, the control accuracy is high enough.
In order to achieve the second object, according to a second aspect
of the present invention, there is provided an electronic control
system for an engine having, an intake manifold connected to the
engine for inducing air and fuel mixture, an airflow meter mounted
on the intake manifold via a throttle valve for measuring an amount
of air induced thereof and for generating an air amount signal, an
exhaust manifold connected to the engine for exhausting burnt
gases, a nitrogen oxide concentration sensor inserted in the
exhaust manifold for detecting a nitrogen oxide amount in the burnt
gases and for producing a nitrogen oxide signal, a crank angle
sensor mounted on the engine for sensing an engine speed and for
generating an engine speed signal, a pressure sensor mounted on the
engine for detecting a combustion pressure in a cylinder and for
outputting a pressure signal, an EGR valve communicated to the
exhaust manifold for recirculating exhaust gases to the intake
manifold and mode setting means for switching an engine operating
mode from an economy mode to a power mode or vise versa, the system
comprising: driving condition determining means, responsive to the
engine speed and pressure signals, for deciding an operating
condition of the engine and for generating an operating condition
signal; combustion fluctuating rate calculating means, responsive
to the pressure signal, for calculating an actual combustion
fluctuating rate and for generating a fluctuating rate signal;
combustion fluctuating rate comparing means, responsive to said
operating condition and the fluctuating rate signals, for deriving
an optimum air-fuel ratio by comparing the fluctuating rate signal
with a standard value stored in a map and for producing a first
control signal; nitrogen oxide gas calculating means, responsive to
the nitrogen oxide signal, for calculating an actual nitrogen oxide
gas amount and for generating a nitrogen oxide signal; exhaust gas
comparing means, responsive to the operating condition signal and
the nitrogen oxide exhaust signals, for judging the optimum
air-fuel ratio by comparing the actual nitrogen oxide gas amount
with a desired nitrogen oxide value stored in a memory and for
producing a second control signal; and emission gas recirculation
rate setting means, responsive to the first and second control
signals, for deciding an optimum EGR rate by referring a target EGR
rate corresponded to each engine operating condition stored in an
EGR map so as to accurately operate the EGR valve in both said
economy and power modes.
According to the second apparatus, the combustion fluctuating rate
is obtained in accordance with the pressure in the cylinder, and
the NOx exhaust rate is obtained in accordance with the
concentration of the NOx in exhaust gas and the intake air
amount.
Subsequently, the combustion fluctuating rate and the limit value
under the present driving condition are compared, while the NOx
exhaust rate and the limit value under the present driving
condition are compared.
If the combustion fluctuating rate is higher than the limit value,
the EGR amount is decreased. The NOx exhaust rate is higher than
the limit value, on the other hand, the EGR amount is increased to
reduce the NOx exhaust.
Thus, the EGR control under each of driving conditions is carried
out on the basis of the combustion fluctuating rate and the NOx
exhaust rate. Accordingly, the restraint of the combustion
fluctuating rate and the reduction of the NOx exhaust rate can be
obtained, so that high-reliability control accuracy can be
attained.
Within the range of the EGR control region, moreover, the driver
can select the EGR rate so that the rate is increased when the fuel
cost is preferential and is decreased when the running performance
is preferential. Thus, the driving performance is available at the
driver's intention, ensuring good convenience of use.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of an air-fuel ratio control apparatus
for a lean-burn engine according to a first embodiment of the
present invention;
FIG. 2 is a diagram showing an NOx exhaust and combustion
fluctuating rate compared with the air-fuel ratio according to the
first embodiment;
FIG. 3 is a flow chart showing an air-fuel ratio control according
to the first embodiment;
FIG. 4 a functional block diagram of an EGR control apparatus
according to a second embodiment of the present invention;
FIG. 5 is a flow chart showing an EGR control sequence according to
the second embodiment;
FIG. 6 is a conceptual diagram showing an EGR control range
according to the second embodiment;
FIG. 7 is a schematic view of the engine according to the second
embodiment; and
FIG. 8 is a circuit diagram of the control apparatus according to
the second embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments of the present invention will become
understood from the following detailed description referring to the
accompanying drawings.
A first embodiment of the present invention will now be described
with reference to the drawings of FIGS. 1 to 3.
Referring to FIG. 1, the general structure of a lean-burn engine
will be described. Numeral 1 denotes an engine body for lean
combustion. In an air induction system of the engine body 1, an air
cleaner 2 communicates with an intake manifold 6 by a duct 3 and a
throttle body 5 which is provided with a throttle valve 4. The
manifold 6 is fitted with an injector 7 for injecting a fuel for
each cylinder. The intake manifold 6 is provided with means (not
shown) for generating swirls or tumbles, whereby swirls or tumbles
are produced in a combustion chamber during air intake, so that a
leaner air-fuel mixture than that with the theoretical air-fuel
ratio is used for combustion.
In the lean-burn engine, moreover, the fuel is so lean that HC and
CO, harmful substances, in the exhaust gas are little, but NOx can
not be decreased, so that it is necessary to decrease NOx. To
attain this, an exhaust manifold 8 is fitted with a lean-NOx
catalytic converter 10 as an exhaust emission control unit. Thus,
the NOx in the exhaust gas is mainly reduced at a high temperature
by a lean-NOx catalyst, so that the exhaust gas is purified. The
catalytic converter 10 communicates with a muffler 9 through an
exhaust pipe 11.
The following is a principle of a control system.
The pressure in the cylinder at an expansion stroke after
combustion is detected to determine the fluctuating state of
combustion, while the NOx concentration of an exhaust system is
detected, and the actual NOx exhaust is calculated. FIG. 2 shows
characteristic curves representing the NOx exhaust and combustion
fluctuating rate compared with the air-fuel ratio (A/F).
When lean control is effected with the air-fuel ratio leaner than
the theoretical air-fuel ratio at 14.7, as shown in FIG. 2, the NOx
exhaust reaches the maximum value when the air-fuel ratio is about
16, and thereafter, decreases gradually as the air-fuel ratio is
lean. Thus, a point a at which the air-fuel ratio is 19 is an
allowable limit of the amount of NOx in the exhaust gas.
On the other hand, the combustion fluctuating rate continues to be
low on the lean side of the air-fuel ratio, and starts suddenly to
become higher when the air-fuel ratio attains about 23. Thus, a
point b at which the air-fuel ratio is 24 is the lean-side limit
for combustion fluctuating.
As seen from these circumstances, it is necessary only that the
air-fuel ratio on the lean side be controlled so as to be adjusted
to the region between the points a and b (i.e., 19 to 24).
The control system will now be described on the basis of the
above-described control principle.
Signals from an airflow meter 12 for detecting an intake air amount
Q and a crank angle sensor 13 for detecting an engine speed N are
applied to the input of a control unit 20. Each cylinder of the
engine body 1 is equiped with a cylinder pressure sensor 14 for
detecting a cylinder pressure P. An exhaust manifold 8 is fitted
with an NOx concentration sensor 15 for detecting the NOx
concentration. Signals from these two sensors are also applied to
the input of the control unit 20.
The control unit 20 includes a driving condition determining unit
21 which receives the engine speed N and the intake air amount Q.
Engine driving conditions are determined in accordance with both
these parameters. Signals for the driving conditions are applied to
the input of an injection quantity calculating unit 22. An
injection quantity Ti is calculated so that a low-NOx lean air-fuel
mixture is obtained, depending on the driving conditions of the
lean-burn engine. The resulting injection signal is delivered to
the injector 7 with a predetermined timing.
The cylinder pressure P and the driving condition signals are
applied to the input of a cylinder pressure detecting unit 23,
whereby the cylinder pressure P for each driving condition is
detected. The cylinder pressure P is applied to the input of a
combustion fluctuating rate calculating unit 24, and an actual
combustion fluctuating rate B is obtained in accordance with the
change of the cylinder pressure P. The driving condition signals
are applied to the input of a combustion fluctuating rate standard
value retrieving unit 25, whereupon a standard value Bmax of the
lean-side limit for each driving condition is retrieved with
reference to a combustion fluctuating rate reference map 26. The
actual combustion fluctuating rate B and the standard value Bmax of
the lean-side limit are applied to the input of a combustion
fluctuating rate comparing unit 27, whereupon they are compared
with each other. If B>Bmax is given, the injection quantity
calculating unit 22 is ordered to enrich the air-fuel mixture.
The NOx concentration and the driving condition signals are applied
to the input of an NOx concentration detecting unit 28, whereupon
the NOx concentration for each driving condition is detected. This
NOx concentration is applied to the input of an NOx exhaust
calculating unit 29, whereupon the intake air amount Q, NOx
concentration NOxconc, and specific gravity .gamma. of NOx are
multiplied together to calculate an actual NOx exhaust A. Also, the
driving condition signals are applied to the input of an NOx
delivery standard value retrieving unit 30, whereupon an
allowable-limit standard value Amax for each driving condition is
retrieved with reference to an NOx delivery standard map 31. The
actual exhausted NOx A and the allowable-limit standard value Amax
are applied to the input of an exhaust gas comparing unit 32,
whereupon they are compared with each other. If A>Amax is given,
the injection quantity calculating unit 22 is ordered to make the
air-fuel mixture lean.
In response to the order for the rich air-fuel mixture based on the
combustion fluctuating rate B or for the lean mixture based on the
NOx exhaust A, the injection quantity calculating unit 22 increases
or decreases the injection quantity Ti to correct it. In the case
where the fuel cost or running performance is preferential, the
injection quantity Ti is generally decreased or increased depending
on the mode. Thus, the air-fuel ratio is controlled so as to be
always kept in the region between the points a and b of FIG. 2. The
control unit 20 determines a proper ignition timing which depends
on the state of operation, in accordance with various input
informations, and delivers an ignition signal to an igniter.
The following is a description of the general operation of the
lean-burn engine.
In operating the engine, air is first induced into the engine body
1 in accordance with the opening of the throttle valve 4. Then,
swirls or the like are generated in the combustion chamber by the
swirl generating means of the intake manifold 6. Depending on each
set of driving conditions based on the intake air amount Q and the
engine speed N, moreover, the injection quantity Ti is calculated
so that the air-fuel mixture is a lean one which substantially
entails a low NOx. This fuel is injected with a predetermined
injection timing by means of the injector 7. A mixture of air and
the fuel in the combustion chamber is ignited by means of an
ignition plug when it is stratified so as to be thicker in the
region near the ignition plug than in the surrounding region as the
swirls are generated. Thus, the lean air-fuel mixture can be burned
satisfactorily, ensuring reasonable fuel cost and satisfactory
driving performance.
Meanwhile, the exhaust gas produced by lean combustion is
discharged from the engine body 1 into the exhaust manifold 8.
Although the amount of unburned HC and CO in the exhaust gas is
small due to the lean-burn air-fuel ratio, in this case, NOx must
be decreased. The exhaust gas containing the NOx is introduced into
the lean-NOx catalytic converter 10, and the NOx is reduced at high
temperature by the lean-NOx catalyst, so that the gas is purified.
The exhaust gas thus purified by the converter 10 is further
discharged in an atmosphere through the muffler 9 in the
down-stream side.
Referring now to the flow chart of FIG. 3, air-fuel ratio control
for the lean-burn engine operation according to the first
embodiment will be described.
First, in Step S1, the driving conditions are determined by the
engine speed N and the intake air amount Q. The cylinder pressure P
is detected in Step S2, and the combustion fluctuating rate B is
calculated in Step S3. In Step S4, the lean-limit standard value
Bmax of the combustion fluctuating rate is retrieved with reference
to the map.
In Step S5, the actual combustion fluctuating rate B and the
lean-limit standard value Bmax are compared with each other. If the
air-fuel ratio exceeds the lean-side limit so that the combustion
fluctuation increases due to a misfire with B>Bmax, the program
advances to Step S6, whereupon the fuel is increased to enrich the
lean air-fuel mixture for correction. Thus, the combustion
fluctuate is restrained in case of the misfire, so that
deterioration of the driving performance can be prevented.
If B.ltoreq.Bmax is given so that the air-fuel mixture is
controlled on the rich side with a less combustion fluctuation, on
the other hand, the program advances from Step S5 to Step S7,
whereupon the actual NOx concentration NOxconc is detected. The NOx
exhaust A is calculated in Step S8, and the allowable-limit
standard value Amax of the NOx exhaust corresponding to the driving
conditions is retrieved in Step S9.
Then, the two values are compared with each other in Step S10. If
the NOx exhaust A exceeds its allowable limit so that the exhaust
gas worsens with A>Amax, the program advances to Step S11,
whereupon the fuel is decreased to correct the air-fuel ratio to
the lean side. Thus, the air-fuel mixture becomes leaner, so that
the NOx exhaust is lessened.
If A.ltoreq.Amax is given, it is decided that the exhaust gas is in
a good condition, and that the air-fuel mixture is within the
proper range between in the points a and b of FIG. 2. In this case,
the program advances from Step S10 to Step S12, whereupon the
preferable mode is selected. If the mode is a fuel-economy mode for
the precedence of fuel cost, the program advances to Step S11,
whereupon the air-fuel mixture is controlled so as to be leaner, so
that the fuel cost is improved to the maximum. If the mode is a
power mode for the precedence of running performance, the program
advances to Step S6, whereupon the air-fuel mixture is controlled
so as to be richer. As a result, the air-fuel mixture becomes
relatively rich, so that vibration and other characteristics are
improved.
Thus, the lean-burn air-fuel ratio of the lean-burn engine is
always controlled within the region between the lean-side limit b
for combustion fluctuation and the allowable limit a of the NOx
exhaust. Accordingly, both the driving performance and the exhaust
gas characteristics can be maintained favorably at the same time.
Since the NOx concentration of the exhaust gas is restricted to the
allowable limit, the lean-NOx catalytic converter 10 of the exhaust
system can always remove the NOx with reliability.
In the lean-burn engine according to the first embodiment, as
described above, the actual NOx concentration of the exhaust gas is
detected to determine the state of NOx exhaust, and the air-fuel
mixture is controlled within the region between the lean-side limit
for combustion fluctuation and the allowable limit of the NOx
exhaust. Accordingly, the driving performance is improved, and the
NOx in the exhaust gas is decreased securely. Since the region for
the air-fuel ratio control is extended on the rich side, moreover,
vibration can be lessened in the power mode.
For each driving condition, furthermore, the combustion fluctuation
is compared with its standard value to determine the operation
state, the NOx exhaust is compared with its standard value to
determine the state of the exhaust gas, and the air-fuel ratio is
controlled on the rich or lean side. Thus, the control accuracy is
high.
Referring now to FIGS. 4 to 8, an engine control apparatus
according to a second embodiment of the present invention will be
described.
FIG. 4 is a functional block diagram of the EGR control apparatus,
FIG. 5 is a flow chart showing an EGR control sequence, FIG. 6 is a
conceptual diagram showing an EGR control range, FIG. 7 is a
schematic view of an engine, and FIG. 8 is a circuit diagram of the
control apparatus.
In FIG. 7, numeral 41 denotes an engine body. An intake manifold 42
communicates with the upstream side of the engine body 1. An
injector 43 is located directly on the upstream side of an intake
valve (not shown) which is attached to an intake port of each
cylinder of the intake manifold 42. A throttle valve 45 is provided
in an intake pipe 44 which communicates with the intake manifold
42. An air cleaner 46 is attached to an intake port of the pipe
44.
An exhaust pipe 48 communicates with the downstream side of the
engine body 41 by an exhaust manifold 47. A muffler 49 communicates
with the downstream side of the exhaust pipe 48, and a catalyst 50
for purifying the exhaust gas is provided in the middle of the pipe
48. The engine according to the illustrated embodiment serves for
the control of the theoretical air-fuel ratio, and a ternary
catalyst is used as the catalyst 50.
An airflow meter 51 for detecting the mass flow of the intake air
is attached to the intake port of the intake pipe 44 of the suction
system. A throttle sensor 52 for detecting the opening of the
throttle valve 45 is located adjacent to the valve 45.
An O.sub.2 sensor 53 for detecting the oxygen concentration of the
exhaust gas and an NOx concentration sensor 54 for detecting the
concentration of nitrogen oxides (NOx), such as NO and NO.sub.2, in
the exhaust gas are interposed between the junction of the exhaust
manifold 47 of the exhaust system and the catalyst 50.
A cylinder pressure sensor 55 is provided for detecting the
internal pressure of a specific cylinder, and a crank angle sensor
56 is opposed to a crank rotor 41b which is mounted on a crank
shaft 41a. The sensor 56 is designed so as to detect protrusions or
the like which are arranged at regular intervals on the outer
peripheral surface of the rotor 41b. The sensor 56 calculates the
engine speed N and ignition timing according to the time intervals
at which the protrusions are detected.
The respective junctions of the exhaust and intake manifolds 47 and
42 communicate with each other by an EGR passage 57. An EGR valve
58 is provided in the middle of the passage 57. When the valve 58
is opened, small amount of the exhaust gas, depending on the
opening degree of the valve 58, is returned to the induction system
and burned again.
As shown in FIG. 8, an ECU 61 is provided with a CPU 62, a ROM 63,
a RAM 64, an oscillator 65, input ports 66a and 66b, and output
ports 66c and 66d. These elements are connected to a microcomputer
via a bus line.
Analog signals from the airflow meter 51, the NOx concentration
sensor 54, and the O.sub.2 sensor 53 are delivered to an A/D
converter 63 through a multiplexer 62. Thereupon, they are
converted into digital signals in the converter 63, and are applied
in succession to the one input port 66a. The waveform of a crank
angle signal from the crank angle sensor 56 is properly shaped in a
shaping circuit 67, and is applied to the other input port 66b. A
signal from the throttle sensor 52 is applied to the other input
port 66b through an input circuit 68, whereupon it is determined
whether the throttle valve 45 is open or fully closed. The peak
value of an output from the cylinder pressure sensor 55 is
waveform-shaped by a shaping circuit 69, and is applied to the
other input port 66b.
Further, the input port 66b is connected with a mode selector
switch 59. By operating the switch 59, a driver can select the
precedence mode between the cost-first mode and the power mode. If
the fuel-economy mode is selected by the selector switch 59, EGR
control is carried out with the maximum value of an EGR rate within
the EGR Control range. If the power mode is selected, on the other
hand, the EGR control is effected with the minimum value of the EGR
rate within the EGR control range.
Moreover, the EGR valve 58 and the injector 43 are connected to the
output ports 66c and 66d through driving circuits 70 and 71,
respectively. The opening of the valve 58 is controlled in response
to a control signal for a predetermined duty ratio outputted from
the ECU 61.
The following is a description of an arrangement for the EGR
control in the ECU 61.
As shown in FIG. 4, the ECU 61 is provided with a cylinder pressure
detecting unit M1 which, based on the output value of the cylinder
pressure sensor 55, detects the peak value of a cylinder pressure
for each cycle or a cylinder pressure for a fixed crank angle
during a combustion stroke. Also, the ECU 61 is provided with an
NOx concentration detecting unit M2 for detecting the NOx
concentration of the exhaust gas in accordance with the output
value of the NOx concentration sensor 54. Moreover, the ECU 61
includes a driving condition detecting unit M3 for detecting engine
driving conditions on the basis of an engine speed N.sub.E, intake
air amount Q, etc.
The ECU 61 is further provided with a combustion fluctuating rate
calculating unit M4 which calculates a combustion fluctuating rate
(D) in accordance with the ratio between the weighted average of
the peak value of the cylinder pressure for each cycle detected by
the cylinder pressure detecting unit M1 and the cylinder pressure
detected this time or the ratio between the weighted average of the
cylinder pressure for the fixed crank angle for each combustion
cycle and the cylinder pressure detected this time.
Moreover, the ECU 61 is provided with a comparing unit M5 which
compares the combustion fluctuating rate (D), calculated by the
combustion fluctuating rate calculating unit M4 with an allowable
limit value (Dmax) of the combustion fluctuating rate (D), which is
set by map retrieval, using as parameters the engine speed N and
engine load (e.g., basic injection quantity obtained in accordance
with the engine speed N and intake air amount Q) detected by the
driving condition detecting unit M3.
Furthermore, the ECU 61 is provided with an NOx exhaust rate
calculating unit M6 for calculating an NOx exhaust rate (C)
according to the NOx concentration of the exhaust gas and the
intake air amount Q.
The ECU 61 is further provided with an NOx exhaust rate comparing
unit M7 which compares the NOx exhaust rate (C), calculated by the
NOx exhaust rate calculating unit M6, with an allowable limit value
(Cmax) of the NOx exhaust rate (C), which is set by map retrieval,
using as parameters the engine speed N and engine load (e.g., basic
injection quantity obtained in accordance with the engine speed N
and the intake air amount Q) detected by the driving condition
detecting unit M3.
Moreover, the ECU 61 includes a driving mode setting unit M8 for
setting the driving mode by determining, from the output value of
the mode selector switch 59, whether the selected mode is the
economy mode or the power mode.
The ECU 61 further includes a target EGR rate setting unit M9 which
sets a target EGR rate when the combustion fluctuating rate
comparing unit M5 concludes that the combustion fluctuating rate
(D) is lower than the allowable limit value (Dmax), and when the
NOx exhaust rate comparing unit M7 concludes that the NOx exhaust
rate is lower than the allowable limit value (Cmax). Also, the unit
M9 sets the target EGR rate with the value of the EGR rate
decreased when the combustion fluctuating rate (D) is higher than
the allowable limit value (Dmax), and sets the target EGR rate with
the value of the EGR rate increased when the NOx exhaust rate (C)
is higher than the allowable limit value (Cmax).
Furthermore, the EGR 61 is provided with the EGR valve driving
circuit 70 which delivers a driving signal corresponding to the
target EGR rate to the EGR valve 58.
Referring now to the flow chart of FIG. 5, the sequence of EGR
control by the ECU 61 will be described.
The flow chart (FIG. 5) shows a routine which is executed for each
predetermined crank angle or each predetermined calculation
period.
First, various data for the engine driving conditions, including
the present engine speed N, the intake air amount Q, etc. are
detected in Step S21. The cylinder pressure for the present
combustion cycle is detected in Step S22, and the combustion
fluctuating rate (D) is calculated on the basis of the ratio
between the weighted average of cylinder pressures for the
individual combustion cycles and the cylinder pressure for the
present combustion cycle in Step S23. In Step S24, on the other
hand, the NOx concentration of the exhaust gas is detected in
accordance with the output signal from the NOx concentration sensor
54. In Step S25, the exhaust rate (C) of the NOx in the exhaust gas
is calculated on the basis of the ratio between the intake air
amount Q and the NOx concentration.
In Step S26, the combustion fluctuating rate (D) is compared with
the allowable limit value (Dmax) which is previously set by map
retrieval using the engine speed N and the engine load as
parameters.
FIG. 6 shows the relationship between the EGR rate and the
combustion fluctuating rate (D) under certain driving conditions.
As shown in FIG. 6, the combustion fluctuating rate (D) tends
suddenly to rise when the EGR rate is increased to a certain level.
The allowable limit value (Dmax) of the combustion fluctuating rate
(D), which is indicated by a point d in FIG. 6, varies depending on
the driving conditions. The map is loaded with the allowable limit
values (Dmax) for varied driving conditions which are previously
obtained through various experiments or the like.
If it is concluded in Step S26 that the combustion fluctuating rate
(D) under the current driving conditions is lower than the
allowable limit value (Dmax), the program advances to Step S27. If
it is concluded that the changing rate (D) is higher than the
allowable limit value (Dmax), on the other hand, the program jumps
to Step S30, whereupon the target EGR rate is set at a
predetermined reduced value, and the routine is finished.
If it is concluded in Step S26 that the combustion fluctuating rate
(D) is lower than the allowable limit value (Dmax), moreover, the
NOx exhaust rate (C) is compared, in Step S27, with the allowable
limit value (Cmax) which is previously set by map retrieval using
the engine speed N and the engine load as parameters.
FIG. 6 shows the relationship between the EGR rate and the NOx
exhaust rate (C) under certain driving conditions. As shown in FIG.
6, the NOx exhaust rate (C) tends to vary substantially in inverse
proportion to the EGR rate and gradually to increases as the EGR
rate is decreased. The allowable limit value (Cmax), which is
indicated by a point c in FIG. 6, varies depending on the driving
conditions. The map is stored with the allowable limit values
(Cmax) for varied driving conditions which are previously obtained
through various experiments or the like.
If it is concluded in Step S27 that the NOx exhaust rate (C) is
higher than the allowable limit value (Cmax), the program jumps to
Step S29, whereupon the target EGR rate is set at a predetermined
increased value, and the routine is finished.
If it is concluded in Step S27 that the NOx exhaust rate (C) is
lower than the allowable limit value (Cmax), on the other hand, the
program advances to Step S28, whereupon it is determined, from the
output signal from the mode selector switch 59, whether the mode
selected by the driver is the economy mode or the power mode.
If the economy mode is selected, the program advances to Step S29,
whereupon the target EGR rate is set at a predetermined increased
value, and the routine is finished. If the power mode is selected,
on the other hand, the program advances to Step S30, whereupon the
target EGR rate is set at a predetermined decreased value, and the
routine is finished.
The control signal for the predetermined duty ratio corresponding
to the target EGR rate is delivered to the EGR valve 58 through the
driving circuit 70.
If the target EGR rate is adjusted to a value such that the EGR
rate increases, the opening degree of the EGR valve 58 is reduced.
As a result, the EGR rate for the reflux to the induction system
with this opening degree of the valve 58 is controlled so as to be
within the range from the allowable limit value (Cmax) to the
allowable limit value (Dmax).
If the driver selects the power mode, the EGR rate is controlled so
as to be reduced within the EGR control range mentioned above. If
the economy mode is selected, on the other hand, the EGR rate is
controlled so as to be increased within the EGR control range. The
driving performance is available at the driver's request.
The present invention may be also applied to lean-burn air-fuel
ratio control. In this case, a lean-NOx catalyst is used as the
catalyst 50, and the O.sub.2 sensor can be omitted.
According to the second embodiment of the present invention, as
described above, the EGR control under each of driving conditions
is carried out on the basis of the combustion fluctuating rate and
the NOx exhaust rate.
Accordingly, the restraint of the combustion fluctuating rate and
the reduction of the NOx exhaust rate can be attained, so that
high-reliability control accuracy can be obtained.
Within the range of the EGR control region, moreover, the driver
can control the EGR rate so that the EGR rate is increased when the
fuel cost is preferential and is decreased when the running
performance is preferential. Thus, the driving performance is
available at the driver's request, ensuring good convenience of
use.
While the presently preferred embodiments of the present invention
have been shown and described, it is to be understood that these
disclosures are for the purpose of illustration and that various
changes and modifications may be made without departing from the
scope of the invention as set forth in the appended claims.
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